This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A336874 #5 Aug 16 2020 12:57:26 %S A336874 11,2,21,212,22,222,1,2221,112,12,122,1221,221,12212,121,1121,1210, %T A336874 220,110,111,113,223,114,224,115,225,226,116,227,228,117,229,2211,3, %U A336874 31,312,32,23,2123,2231,2122,2120,118,119,22111,1111 %N A336874 The self-sandwiches sequence (see Comments lines for definition). %C A336874 Imagine we would have a pair of adjacent integers in the sequence like [1951, 2020]. The sandwich would then be made of the rightmost digit of a(n), the leftmost digit of a(n+1) and, in between, the single digit d of the sequence itself not been yet duplicated inside a sandwich. The pair [1951, 2020] would then produce the sandwich 1d2. Please note that the pair [2020, 1951] would produce the genuine sandwich 0d1 (we keep the leading zero: these are sandwiches after all, not integers). %C A336874 Now we want the sequence to be the lexicographically earliest sequence of distinct positive terms such that the successive sandwiches emerging from the sequence rebuild it, digit after digit. %C A336874 The authors are unable to compute more terms than the ones proposed here and ask the readers' indulgence. %e A336874 The first successive sandwiches are: 112, 212, 122, 222, 212,... %e A336874 The 1st one (112) is visible between a(1) = 11 and a(2) = 2; we get the sandwich by inserting the 1st digit of the sequence itself, 1. %e A336874 The 2nd sandwich (212) is visible between a(2) = 2 and a(3) = 21; we get this sandwich by inserting inserting the 2nd digit of the sequence itself, 1. %e A336874 The 3rd sandwich (122) is visible between a(3) = 21 and a(4) = 212; we get this sandwich by inserting the 3rd digit of the sequence itself, 2. %e A336874 The 4th sandwich (222) is visible between a(4) = 212 and a(5) = 22; we get this sandwich by inserting the 4th digit of the sequence itself, 2. Etc.. %e A336874 The successive sandwiches rebuild, digit by digit, the starting sequence. %Y A336874 Cf. A335600 (first sequence of this kind, linked to many others). %K A336874 base,nonn %O A336874 1,1 %A A336874 _Eric Angelini_ and _Carole Dubois_, Aug 06 2020